A metal manifold constructed of a ninety-percent-tantalum ten-percent-tungsten refractory alloy (Ta-10W) is coated by spraying a slurry onto the exterior surfaces, and by filling with slurry and draining to coat the interior surfaces. The coated manifold is then placed in a vacuum oven for approximately 45 minutes at approximately 2550.degree. F., causing the unfired (i.e., green) slurry to undergo a fusion reaction producing a ceramic coating of complex silicides. During system operation at higher temperatures (i.e., 3000.degree. F.) the complex silicides form a protective barrier between the metal manifold and the oxidizing environment of hot gases.
A prior-art slurry (formerly used to protect the surfaces of a metal manifold from 3000.degree. F. gases) was comprised of three metal powders; sixty percent silicon, thirty percent hafnium, and ten percent chromium (the foregoing percentages are by weight, and with respect to the total weight of the three metal powders). In addition to the metal powders, the prior-art slurry was comprised of a nitrocellulose lacquer resin binder and an antisettling agent sold under the trademark MPA 60 by Rheox Inc., a subsidiary of NL Industries. The function of a binder is to cause the metallic powders to adhere to the surface being coated during the coating process. The function of an antisettling agent is to keep the metal powders uniformly suspended in the slurry both during storage and during the application process. Further details concerning the composition of the prior-art slurry are set forth in Naval Ordnance Command Weapon Specification 17302 (NAVORD WS 17302), entitled Material Specification, Slurry, Hafnium Silicide, which is incorporated herein by reference.
A problem with the coating formed from the above prior-art slurry was that when the manifold was in operation (i.e., conducting the 3000.degree. F. gases) liquid silicides of Hf and Cr formed that sloughed off from the coating, and interfered with downstream moving parts. Another problem associated with the prior-art slurry was that the ingredient hafnium is an expensive and potentially hazardous material. An inconvenience in processing was that the MPA 60 antisettling agent (a waxy paste) was difficult to mix into the slurry.
Hence there is a need for a safer and less expensive high temperature protective slurry that would also minimize the risk of interfering with the operation of moving parts (in a hot gas distributing system downstream from the manifold) due to the sloughing away of coating constituents.